CATE validation - uplift uniform confidence bands

[amarvenu]

Added additional functionality to the DRTester validation class to include uniform confidence bands (both two- and one-sided) for uplift curve plots, calculated via the multiplier bootstrap.

Modified files are:

• econml/validate/* (main code files)
• econml/tests/test_drtester.py (test cases)
• notebooks/CATE validation.ipynb (example usage notebook)

Test cases were extended to include the modifications, maintaining 100% coverage:

Looking forward to any comments.